High-frequency protective circuits



Feb. 28, 1950 SPENCER 2,498,719

HIGH-FREQUENCY PROTECTIVE CIRCUITS Filed June 7, 1947 Patented Feb. 28, 1950 HIGH-FREQUENCY PROTECTIVE CIRCUITS Percy L. Spencer, West Newton, Mass, assignor to Raytheon Manufacturing Company, Newton, Mass, a corporation of Delaware Application June 7, 1947, Serial No. 753,180

8 Claims. (Cl. 25017) This invention relates to electrical circuits, and more particularly to a system for automatic protection of a microwave oscillator producing continuous waves.

An object of this invention is to devise a system for automatic protection of a microwave oscillator, such as a magnetron, against high-voltage breakdown at the window or seal thereof.

Another object is to devise a protective system which will automatically afford protection against standing wave amplitudes, in a radio-frequency transmission line, which are in excess of a predetermined value.

A further object is to provide a protective system for a microwave transmission system which will afford protection against an unduly high ratio between standing wave maxima and minima therein.

The foregoing and other objects of the invention will be best understood from the following description of an exemplification thereof, reference being had to the accompanying drawing, wherein the single figure is a diagrammatic representation of a protective system according to this invention.

Now referring to the drawing, the numeral I generally designates an electron-discharge device of the so-called multicavity magnetron type, This device includes an anode 2 and acathode 3. When device I is subjected to a suitable magnetic field and a high direct voltage is impressed between its anode and cathode, there are produced oscillations of a frequency determined by the internal structure of the device. Device I is designed to be capable of producing continuous-wave oscillatory energy in the microwave region of the electromagnetic spectrum.

In order to impress the desired high direct voltage between anode 2 and cathode 3, to energize the magnetron and to thereby cause it to act as a source of radio-frequency energy, cathode 3 is connected, by means of a lead 4, to the negative side of a suitable direct current source, while anode 2 is connected, by means of a lead 5, through the normally-closed contacts 6 of a relay 1, to the positive side of said direct current source. As long as contacts 6 are closed, magnetron I is energized, and microwave electromagnetic energy is continuously generated thereby, but when contacts 6 are opened, the operatin voltage is removed from said magnetron, and such energy is no longer produced.

The microwave energy is picked up by a loop 8 and transmitted through a coaxial line 9 to a hollow metallic waveguide III, the inner-conductor 55 of said line 9 serving as the exciting probe for said guide. Waveguide I0 may be termed a transmission line, since radio-frequency energy is capable of being propagated therealong. The end wall Illa of waveguide It nearest the magnetron I may be termed the window or seal of the magnetron, since, ordinarily, a seal is provided thereat.

Transmission line or waveguide I0 may have any desired length, and a first coupling loop II extends into the interior of said guide, through a suitable opening I2, at a point spaced a distance of an integral number of quarter-wavelengths from the end wall Illa of said guide, or from the window or seal of magnetron I, the term wavelength referring to the predetermined wavelength of the oscillations of oscillator l. The end of loop II which extends through opening I2 to the exterior of the guide I0 is connected to one electrode or element I3 of a discharge device I4 of the cold-cathode gaseous diode type, such as an ordinary neon bulb, the second element of device I4 being denoted by I5.

A second coupling loop I6 extends into the interior of guide I0 through a suitable opening II, loop I6 being located at a point spaced a distance of n quarter-wavelengths beyond loop II in the direction therefrom opposite from wall Illa of the guide, where n is an odd integer. The end of loop I6 which extends through opening I! to the exterior of guide Ill is connected to the electrode I5 of device I4. Thus, the radio-frequency energy of the transmission line or waveguide I0 is coupled to device I4.

Although discharge device It is shown as a diode, gaseous tubes of the multiple-element type may also be used in this invention.

One side of a suitable source of direct current I8, such as a battery, is connected, by means of a lead I9, to electrode i5, the opposite side being connected, by means of a lead 20, through the operating winding 2! of relay 1', to the other electrode I3. A series control circuit is therefore provided, consisting of battery I3, relay winding 2|, and the interelectrode path i3--I5. When the gaseous atmosphere in device I4 is ionized,

current can flow from source I8 through winding 2I and current path I3I5 in series, energizing relay I to open its contacts 6 and deenerg-izing magnetron I.

Device I4 is arranged to be non-conducting except when the gaseous atmosphere thereof is ionized by radio-frequency energy, so that relay 1 is normally deenergized and contacts 6 are normally closed.

Radio-frequency energy of a predetermined wavelength is supplied by source I to waveguide l and is propagated therealong to some reactive load (not shown), causing standing waves to be established in the radio-frequency transmission system including waveguide I0, since continuouswave energy is being propagated along said guide. It is desirable to limit the amplitude of the standing waves in the guide to a certain value, in other words, to in effect maintain the standing wave ratio in the radio-frequency transmission line below a predetermined value, in order to prevent high-voltage breakdown at the window or seal of the tube or source I. This standing wave amplitude may be conveniently limited by limiting the ratio between the standing wave maxima and minima in the guide I0.

Since loops H and I6 are spaced an odd number of quarter-wavelengths apart, the standing wave maxima may be considered as coupled to one loop and the standing wave minima to the other. When the ratio between said maxima and said minima becomes high enough to produce a voltage equal to or greater than the ionization voltage of device I4, the gaseous atmosphere in said device becomes ionized, or said device breaks down, allowing current from the source I8 to pass through the now conducting path I3-I5 of said device to energize relay I, opening contacts 6 to deenergize magnetron I.

It is thus apparent that when the ratio between standing wave maxima and standing wave minima in the guide I0 is above a predetermined value, the magnetron I is automatically deenergized. The discharge device I4 becomes ionized by radio-frequency energy in the guide II], but this ionization can occur only when the ratio between standing wave maxima and minima in the guide is above a value which is predetermined by the ionization voltage of device I4. Obviously, the source I8, in addition to providing operating current for relay 1, provides also a voltage across the electrodes I3 and I5 when the discharge device I4 is in a nonconducting state. This voltage biases the device I4, so that the voltage coupled thereto by the loops II and I6 is actually in addition to a predetermined bias.

Tubes of the magnetron type have the inherent characteristic that, for stable operation of the tube, there is an optimum standing wave phase value which is desired at the window or seal thereof. In other words, for stable operation the tube wants to look at either a standing wave maximum or a standing wave minimum at the seal thereof. It is to be understood that reliable operation of the system of this invention is obtained only when the standing wave phase in the transmission line It is fixed and known approximately, and when the coupling loop II is spaced a distance from end wall Illa such that it is placed in the vicinity of a standing wave voltage maximum or a standing wave voltage minimum, depending on the phase that the tube I wants to see in order to operate in a stable manner.

Since the factor n is an odd integer, when loop II is at a minimum, loop I6 will be at a 4 predetermined value, thus also protecting magnetron I against high-voltage breakdown at the window or seal thereof.

Of course, it is to be understood that this invention is not limited to the particular details as described above, as many equivalents will suggest themselves to those skilled in the art. For example, alternating voltage might be used in the control system of this invention, in place of source I8, in which case an alternating voltage relay would be required in the controlled part of the system. Various other variations will suggest themselves. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of this invention within the art.

What is claimed is:

1. A protective system comprising a source of radio-frequency energy of a predetermined wavelength, a radio-frequency transmission line connected to the output of said source, an electrical discharge device connected in series with a source of potential and a circuit-controlling means, said discharge device having a pair of electrodes which cause said device to conduct upon the application thereto of a voltage in excess of a predetermined value, said electrodes being coupled to said line at points separated by substantially an odd number of quarter- Wavelengths, said circuit-controlling means cutting off the supply of radio-frequency energy from said transmission line upon actuation by conduction through said device, whereby said supply is cut off from said transmission line upon the occurrence of standing wave peaks on said transmission line in excess of a predetermined value.

2. A protective system comprising a source of radio-frequency energy of a predetermined wavelength, a radio-frequency transmission line connected to the output of said source, a gas-filled electrical discharge device connected in series with a source of potential and a circuit-controlling means, said discharge device having a pair of electrodes which ionize the gas in said device and cause said device to conduct upon the application thereto of a voltage in excess of a predetermined value, said electrodes being coupled to said line at points separated by substantially an odd number of quarter-wavelengths, said circuit-controlling means cutting off th supply of radio-frequency energy from said transmission line upon actuation by conduction through said device, whereby said supply is cut oil from said transmission line upon the occurrence of standing wave peaks on said transmission line in excess of a predetermined value.

3. A protective system comprising a source of radio-frequency energy of a predetermined wavelength, a radio-frequency transmission line connected to the output of said source, a gas-filled electrical discharge device having a pair of electrodes connected in series with a source of potential and a circuit-controlling means, said pair of electrodes ionizing the gas in said device and causing said device to conduct upon the application thereto of a voltage in excess of a predetermined value, said electrodes also being coupled to said line at points separated by substantially an odd number of quarter-wavelengths, said circuit-controlling means cutting oil? the supply of radio-frequency energy from said transmission line upon actuation by con [5 duction through said device, whereby said sup ply is cut off from said transmission line upon the occurrence of standing wave peaks on said transmission line in excess of a predetermined value.

4. A protective system comprising a source of radio-frequency energy of a predetermined wavelength, a hollow waveguide connected to the output of said source, an electrical discharge device connected in series with a source of potential and a circuit-controlling means, said discharge device having a pair of electrodes which cause said device to conduct upon the application thereto of a voltage in excess of a predetermined value, said electrodes being coupled to said waveguide at points separated by substantially an odd number of quarter-wavelengths, said circuitcontrolling means cutting ofi the supply of radiofrequency energy from said waveguide upon actuation by conduction through said device, whereby said supply is cut off from said waveguide upon the occurrence of standing wave peaks on said transmission line in excess of a predetermined value.

5. A protective system comprising a source of radio-frequency energy of a predetermined Wavelength, a hollow waveguide connected to the output of said source, a gas-filled electrical discharge device having a pair of electrodes connected in series with a source of potential and a circuit-controlling means, said pair of electrodes ionizing the gas in said device and causing said device to conduct upon the application thereto of a voltage in excess of a predetermined value, said electrodes also being coupled to said waveguide at points separated by substantially an odd number of quarter-wavelengths, said circuit-controlling means cutting off the supply of radio-frequency energy from said waveguide upon actuation by conduction through said device, whereby said supply is cut off from said waveguide upon the occurrence of standing wave peaks on said transmission line in excess of a predetermined value.

6. A protective system comprising a source of radio-frequency energy, controllable means for supplying an operating voltage to said source, a transmission line connected to said source, a gaseous-discharge device having a plurality of elements, said device being coupled to said line in such away that the gaseous atmosphere of said device is ionizable by standing wave peaks in said line to provide a current-conducting path between said elements, means independent of said transmission line for biasing said device so that said ionization can occur only above a predetermined value of standing wave ratio in said line, and means coupling said device to said controllable means for controlling the same to disconnect said voltage from said source when said device becomes ionized.

'7. A protective system comprising a source of radio-frequency energyoi a predetermined wavelength, controllable means for supplyin an operating voltage to saidsource, a transmission line connected to said source, a gaseous-discharge device having a plurality of elements, two of said elements being coupled to said line at points spaced substantially an odd number of quarters of said wavelength apart in such a way that the gaseous atmosphere of said device is ionizable by standing wave peaks in said line to provide a current-conducting path between said elements, means for -.bias ing said device so that said ionization can occur only when the ratio between standing wave maxima and minima in said line is above apredetermined value, and means coupling said device to said controllable means for controlling the same.

8. A protective system comprising a source of radio-frequency energy} a circuit including a pair of relay contacts for disconnectin an operating voltage from said source when the operating coil of said relay is energized, a transmission line connected ,to said source, a gaseousdischarge device having a plurality of elements, said device being coupled to said line in such a. way that the space between said elements in said device is ionizable by standing wave peaks in said line to provide a current-conducting path between said elements, a voltage and current source for biasing said .device so that said ionization can occur only above a predetermined value of standing wave ratio in said line, and means connecting the space between two of said elements in series between said current source and the operating coil of said relay.

PERCY L. SPENCER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER. anr'nnnncns Radio News, "A Resonant Cavity Wavemeter, article by McQuay, pp. '36 and 37,-Febn1ary 1946. 

